The present invention relates to a method for processing the sludge, that inevitably forms during halogen-type electrolytic tinplating. In particular, the present invention relates to a method for recovering metallic tin at a high yield and at a high purity from the sludge. Further, the present invention relates to a method for processing sludge generated by halogen-type electrolytic tinplating in which other useful materials can be recovered for reuse from the filtrates and the precipitates resulting from the process. 2. Description of the Related Art
Halogen-type electrolytic tinplating producing is one of the continuous manufacturing methods for tin-electroplated steel sheets. The halogen-type electrolytic tinplating process uses a halide electrolytic bath which contains for example a hydrogen halide solution such as hydrochloric acid, stannous chloride, sodium chloride, sodium fluoride, sodium hydrogen fluoride and the like. By electroplating using a combination of metallic tin as an anode and an advancing steel sheet as a cathode, metallic tin is electrodeposited on the surface of the steel sheet.
A halogen-type electrolytic tinplating process using an acidic electrolyte (plating solution) is advantageous in that the metallic tin at the anode can be dissolved as available stannous ions. However, some of the stannous ions are simultaneously oxidized by oxygen to stannic ions, which results in the formation of a large amount of sludge. The resulting sludge contains sodium hexafluorostannate (Na.sub.2 SnF.sub.6) as one of the main components. The sludge also contains a small amount of sodium hexafluoroferrate (Na.sub.3 FeF.sub.6) which includes ferric ions dissolved from the steel sheet.
Sludge is also formed by ferrous ions dissolved from the steel sheet into the plating solution. Although the halogen-type electrolytic tinplating process permits high current density operation and is suitable for a high-speed production line, the high-speed advancement of the steel sheet causes agitation of the plating bath and thus introduction of air into the bath. Currents in the plating bath also cause introduction of air into the bath. Ferrous ions are consequently oxidized by oxygen dissolved into the plating bath to form ferric ions. In the plating bath, the resulting ferric ions are reduced to ferrous ions by causing stannous ions to be oxidized to stannic ions, which results in the formation of sludge. In order to prevent oxidation of stannous ions, sodium ferrocyanide (Na.sub.4 Fe(CN).sub.6) is added. Sodium ferrocyanide reacts with ferric ions to form ferric ferrocyanide (Fe.sub.4 [Fe(CN).sub.6 ].sub.3) which precipitates in the sludge.
Accordingly, the sludge in the plating bath contains cyanides and fluorides after halogen-type electrolytic tinplating of the steel sheet. Sludge primarily containing Na.sub.3 FeF.sub.6 and Fe.sub.4 [Fe(CN).sub.6 ].sub.3 is called "blue sludge", whereas sludge primarily containing Na.sub.2 SnF.sub.6 is called "white sludge", based on their respective colors. For the purpose of a clearer explanation of the present invention, the blue sludge and its reaction product may be referred to as "iron-containing sludge", and the white sludge and its reaction product may be referred to as "tin-containing sludge". Increasing amounts of the blue and white sludges retard the electroplating operation, hence the operation must be suspended to remove the sludge.
Since the sludge contains useful tin, the tin is recovered as a metal by a conventional recovery process as shown in FIG. 5. Such a recovery process, however, consists of many preliminary steps before electrolysis, and thus is economically disadvantageous.
Japanese Kokai No. 57-70242 discloses a method for recovering metallic tin from sludge generated by a halogen-type electrolytic tinplating process, as shown in FIG. 4. In this method, the sludge is converted to a slurry, hot alkali (i.e. NaOH) is added to the slurry, and blue sludge containing large amounts of iron is separated by filtration. On the other hand, acid is added to the filtrate containing white sludge components to adjust the pH to 7 to 13, and the filtrate is subjected to electrolysis in order to recover metallic tin by electrodeposition.
That method, however, has the following disadvantages. When the hot alkali is added to the slurry, some of the tin sludge is deposited together with the iron sludge. Thus, the tin content in the filtrate is decreased, resulting in a reduced tin yield recovered from the filtrate in the subsequent steps. The recovered metallic tin has a relatively low purity of approximately 99.5%. Such low-purity tin does not satisfy the quality requirements for an anode for tin plating of steel sheets for cans. Moreover, since the method aims only at recovery of metallic tin, the solution containing large amounts of nonrecoverable fluorine components is wasted.
Japanese Kokai No. 9-103790 discloses another method for recovering tin. Sludge is subjected to leaching with acidic water (pH 5.5 to 6) containing an oxidizing agent to remove blue sludge. The pH of the filtrate is adjusted to 7.5 to 9.0 to precipitate tin hydroxide or tin oxide hydrate. The precipitate is then reduced to metallic tin.
The reduction processes proposed in this method include a molten-salt reduction process and a smelting reduction melting process in which the precipitate is melted with graphite and the resulting iron is separated based on the difference in the melting point. These reduction processes are dry processes and involve formation of fine particles, resulting in environmental hazards during the operation. Furthermore, the melting point of the precipitate changes depending on the iron content in the precipitate; therefore the temperature and the reduction are controlled only with great difficulty. As a result, the purity and yield of tin decrease.
Japanese Kokai No. 9-67699 discloses a sludge processing apparatus for performing a process as shown in the flow chart of FIG. 3. The apparatus includes a sludge-separating unit, a white-sludge processing unit, and a blue-sludge processing unit. The sludge-separating unit separates the initial sludge into an aqueous white-sludge solution containing stannous ions and a blue-sludge solid content. The white-sludge processing unit recovers metallic tin from the aqueous solution, whereas the blue-sludge processing unit decomposes the solid content into harmless compounds which can be safely disposed of. In this method, NaOH is added to the aqueous solution containing stannous ions to form white sludge primarily containing SnO.sub.2.nH.sub.2 O. The white sludge is heated with a carbon reducing agent to form a metallic tin melt, while iron is removed based on the difference in its melting point. The remaining tin is cast as an ingot.
This method therefore also uses a difference in the melting point for separating tin from iron, as in Japanese Kokai No. 9-103790. Thus, this method has disadvantages of unsatisfactory separation and environmental pollution due to the formation of fine particles. Furthermore, incorporation of the fine particles into the recovered tin results in a low purity and a low yield of tin. Both the molten-salt reduction process and the smelting reduction melting process require expensive facilities, and thus have economic disadvantages. In Japanese Kokais Nos. 9-103790 and 9-67699, the major part of the filtrate formed by sludge processing is disposed of without recovery of useful components.
The present inventors disclose a method for separating the initially-formed sludge into blue sludge and white sludge and recovering useful components from the waste solution, in Japanese Kokai No. 10-72629. Tin obtained by this method, however, is still insufficiently pure for use an anode for tin plating. This method would also benefit from improvement in the yield of the recovered tin and workability, and simplification of the process in view of process cost reduction.